The present invention relates to an image forming apparatus that cools sheets that have passed through a fixing unit.
Image forming apparatuses employing electrophotography include, for example, multifunction peripherals, printers, copiers, and facsimile machines. An electrophotographic image forming apparatus includes a fixing device. The fixing device fixes toner to a sheet. The sheet that has undergone fixing is discharged onto a discharge tray. If sheets are stacked while they are hot, the discharged sheets may stick to each other. To prevent that, sheets are often cooled by being exposed to wind. There are known techniques for cooling sheets as described below.
There is known a printing device (sheet discharging mechanism) that discharges printed sheets onto an external stacker through a discharge opening, wherein the device includes a blowing nozzle through which wind can be blown onto, over the entire width of, the printed sheets discharged through the discharge opening and a blower for sending wind to the blowing nozzle. This structure is intended to sufficiently cool the sheets that are conveyed.
For the fixing of toner, a sheet is heated in a fixing device up to a temperature at which the toner melts. The sheet is then discharged onto a discharge tray. As sheets are stacked on the discharge tray, the weight (pressure) of upper sheets acts on lower sheets. Stacking a large number of printed sheets on the discharge tray may cause toner to stick to adjacent sheets under the temperature and pressure of the sheets. Thus, sheets can stick to each other. In duplex printing, a sheet passes through the fixing device twice. The amount of heat applied to a sheet is larger in duplex printing than in simplex printing. Sheets are more prone to stick together in duplex printing than in simplex printing. Thick sheets store a larger amount of heat than average sheets. With thick sheets, toner is often fixed at a higher temperature. The higher the temperature at which toner is fixed, the more prone sheets are to stick together.
In addition, recent image forming apparatuses adopt toner with lower melting points than ever. Lowering the melting point of toner helps reduce the output of a heater in the fixing device. This helps save energy (reduce power consumption). Low-melting-point toner sticks to adjacent sheets at a lower temperature than does conventional high-melting-point toner. Thus, the lower the melting point of toner, the more prone sheets stacked on the discharge tray are to stick together.
To prevent sheets on the discharge tray from sticking together, cooling wind is often blown onto the sheets. Thereby the sheets are cooled. Thick sheets storing a large amount of heat or sheets printed on both sides require a larger amount of wind than do thin sheets. On the other hand, blowing a large amount of cooling wind to thin sheets causes them to flutter. This makes a jam more likely, and may cause thin sheets to be discharged unevenly onto the discharge tray. A problem here is that the amount of cooling wind has to be one that is adequate to the thickness of sheets and the amount of heat applied to sheets.
Here, a motor is used to rotate a fan. It is common to adjust the amount of wind by varying the voltage applied to the motor. However, with a voltage equal to or lower than 30% to 40% of the rated voltage, the motor may not start (the fan may not rotate). A problem here is that, through the control of the applied voltage alone, the adjustment width of the amount of wind is narrow, that is, the ratio of the maximal amount of wind to the minimal amount of wind is low).
With the known techniques mentioned above, it is possible to blow wind onto, over the entire width of, sheets. However, no consideration is given to adjusting the amount of wind to suit the situation. Thus, they do not solve the problems mentioned above.